1. Field of the Invention
The invention relates to a control device, more particularly, relates to a control device for a vehicle that includes a plurality of power sources that drives a plurality of wheels, and to a control method thereof.
2. Description of the Related Art
A vehicle that includes a plurality of power sources that drives a plurality of wheels, and a control device for such a vehicle are known. For example, Japanese Patent Application Publication No. JP-A-2001-171378 discloses a control device for a four-wheel drive vehicle in which either group of the front wheels and the rear wheels are capable of being driven by a first motor and the wheels of the other group are capable of being driven by a second motor.
This control device finds a target drive force on the basis of the degree of driver's operation of output operating means, and of the vehicle speed. Then, the control device controls the front wheel drive force and the rear wheel drive force for outputting the target drive force from the front wheel side and the rear wheel side on the basis of the state of the vehicle or the operation state of the vehicle.
In the four-wheel drive vehicle as described above, the front wheel drive force and the rear wheel drive force are determined by distributing the target drive force to the front wheel side and to the rear wheel side. The drive forces based on this distribution are transmitted to the front wheels and the rear wheels. However, in the four-wheel drive vehicles as described above, the rate of change of the drive force against time is restricted with a certain constant value, taking into consideration the vehicle stability and the influence on the vehicle behavior when the drive force is transmitted to the front wheels and to the rear wheels. In some cases, the rate of change of the drive force is different between the front wheels and the rear wheels.
If the drive force is controlled in this way, it can happen that the drive force at the time of accelerating or decelerating the vehicle is different in sign between the front wheels and the rear wheels (e.g., the sign of the front wheel drive force is negative while the sign of the rear wheel drive force is positive). For example, in the case where the target drive force changes from a negative value to a positive value in order to accelerate the vehicle, the front wheel drive force and the rear wheel drive force also change from negative values to positive values. If the rear wheel drive force changes faster than the front wheel chive force, it is more likely to happen that the front wheel drive force remains a negative value although the rear wheel drive force is a positive value.
If the sign of the torque is different between the front wheels and the rear wheels, and the torque difference is large between the front wheels and the rear wheels, it is conceivable that the vehicle stability will decline, for example, in the case where the vehicle is turned on a road whose friction coefficient μ is extremely low. However, Japanese Patent Application Publication No. JP-A-2001-171378 does not disclose a concrete means of solution to this problem.